A hollow fiber membrane has been widely used for industrial applications such as microfiltration and ultrafiltration. As to the material used in the membrane, polyethylene, cellulose acetate, polysulfone, polyvinylidene fluoride, polycarbonate, polyacrylonitrile, or the like has been used. A conventional hollow fiber membrane formed of such a material has been developed mainly aiming at improving filtration performance. Therefore, as a conventional hollow fiber membrane exhibits low breaking stress and elongation at break, the hollow fiber membrane breaks due to a rapid temperature change or a pressure fluctuation when shifting to backwash.
Various attempts have been made to solve this problem. As suggested by the invention disclosed in Japanese Patent Application Laid-open No. 59-228016, a method wherein the polymer density of the entire hollow fiber membrane is increased by increasing the polymer concentration in the membrane-forming solution may be generally considered. This method improves the strength of the membrane, but decreases the pore size of the membrane and considerably decreases the water permeability of the membrane. As a result, a hollow fiber membrane having well-balanced strength and water permeability has not as yet been obtained.
The pore size of the membrane is generally increased in order to improve water permeability of the membrane. However, an increase in the pore size generally decreases the fractionation (cutoff) performance and the strength of the membrane.
As described above, a high-performance hollow fiber membrane having well-balanced strength, water permeability, and fractionation performance has not been obtained by a conventional technology. For example, Japanese Patent Application Laid-open No. 04-260424 proposes a method for producing a membrane having high strength and excellent water permeability. However, since the membrane obtained by this method has a large pore size, the water permeability and fractionation performance is not well-balanced.
Japanese Patent Application Laid-open No. 02-102722 discloses a hollow fiber microfiltration membrane in which the pore size is continuously decreased from the outer surface toward the inside of the membrane, is minimized inside the membrane, is continuously increased again toward the inner surface, and is open on the inner surface. However, when filtering liquid or the like from the side of the hollow section (inner surface side) of a membrane having this structure, filtration cannot be stably performed for a long period of time due to the occurrence of rapid clogging.
Japanese Patent Application Laid-open No. 58-155865 discloses a hollow fiber membrane having a dense layer on at least one surface side of the hollow fiber membrane and a porous layer inside the hollow fiber membrane. Japanese Patent Application Laid-open No. 58-155865 discloses a hollow fiber membrane made of a vinyl alcohol polymer, but does not disclose a membrane material comprising a hydrophobic polymer and a hydrophilic polymer. If the hydrophilic polymer is included in the hydrophobic polymer, the molecular chains of the hydrophobic polymer become poorly entangled with each other, whereby a high strength may not be obtained. Moreover, since the hollow fiber membrane made of a vinyl alcohol polymer disclosed in Japanese Patent Application Laid-open No. 58-155865 has a structure in which the dense layer is formed on the outer surface of the membrane, filtration cannot be stably performed for a long period of time due to the occurrence of clogging when a liquid or the like is filtered from the side of the hollow section (inner surface side) of the membrane.
The applicant of the present invention has provided a hollow fiber membrane comprising a hydrophobic polymer and a hydrophilic polymer and having a sponge structure in which the pore size is continuously decreased from the outer surface to the inner surface of the membrane. However, this membrane can be merely used for blood dialysis or ultrafiltration which does not substantially cause albumin to pass therethrough, and is not suitable for plasma purification (Japanese Patent Application Laid-open No. 11-309355, Japanese Patent No. 3281364, and Japanese Patent No. 3281363).
As described above, a hollow fiber membrane for plasma purification which is well-balanced, exhibits a desired strength, water permeability, and fractionation performance, and which rarely clogs even when filtering liquid from the side of the hollow section (inner surface side), has not yet been provided.